1. Field of Endeavor
The present invention relates to a method for starting a combined cycle power plant.
2. Brief Description of the Related Art
With reference to FIG. 1, a combined cycle power plant 1 includes a gas turbine unit 2 and a steam turbine unit 3 (water heater steam cycle including HRSG and steam turbine).
In particular the steam turbine unit 3 has a heat recovery steam generator (HRSG) 4 into which the flue gases, discharged from the gas turbine unit 2, are fed.
In the heat recovery steam generator (HRSG) 4, steam is generated that is then fed into the steam turbine 5.
The steam turbine 5 includes a stop valve 6 (this is an on/off valve), a control valve 7 (they can also be made in one single element or the stop valve 6 can be upstream of the control valve 7), a rotor 8 and guide vanes 9.
In addition, attemperators 10 are provided within the heat recovery steam generator 4 and/or between the heat recovery steam generator 4 and the steam turbine 5, to control the temperature of the steam that comes from the heat recovery steam generator (HRSG) 4 and is supplied into the steam turbine 5.
In order to start a combined cycle plant 1 like the one described, U.S. Pat. No. 7,621,133 discloses a method that includes:                loading the gas turbine unit 1 at its maximum rate,        supplying steam to the steam turbine 5 at a prefixed constant temperature from the initial steam admission until all the steam is admixed into the steam turbine 5.        
In other words, when the combined cycle plant 1 is started, and after the gas turbine unit 2 is loaded, the steam attemperators 10 are regulated such that the steam at a position P1 upstream of the steam turbine 5 (i.e., downstream of the reheat boiler 4 and upstream of the control valve 7) has a substantially constant prefixed temperature.
Nevertheless, when the steam turbine unit 3 is started, the control valve 7 is regulated from a closed position to a fully open position; during this regulation the steam, when passing through the control valve 7, cools because of: the so called Joule-Thompson effect or throttling effect; it causes steam cooling without heat exchange between the steam and control valve 7; and heat exchange between the steam and control valve 7.
Therefore during the control valve 7 regulation, the steam temperature at a position P2 downstream of the control valve 7 and upstream of the rotor 8 is lower than the prefixed temperature.
With reference to FIG. 2, the steam temperature Ts run during a traditional starting is shown (t indicates the time from initial steam admission into the steam turbine 5, i.e., from when the control valve 7 opening starts).
In this figure solid line 15 indicates the steam temperature at the position P1 and dashed line 16 indicates the steam temperature at the position P2.
It is clear that during the time interval 17 (i.e., when the control valve 7 is regulated), because of the already cited Joule-Thompson effect, the steam temperature at the position P2 is generally lower than the temperature at the position P1; in particular the temperature at the position P2 increases from a minimum temperature until it is substantially equal to the temperature at the position P1 (in this condition the control valve 7 is fully open, no cooling for Joule-Thompson effect occurs and the heat exchange between the steam and control valve 7 is typically low).
When the control valve 7 is fully open (i.e., from the end of time interval 17 onwards), the temperatures at the positions P1 and P2 are substantially the same.
The stress to which the rotor 8 is subjected during starting mainly depends on: heat transfer (i.e., steam mass flow and pressure); and steam temperature increase.
Since the temperature of the steam upstream of the rotor 8 (i.e., at the position P2) increases at the beginning of the start up, the heating of the rotor and other thick walled components (such as the turbine casing) is not optimal and results in thermal stresses that limit the number of starting procedures that can be carried out or significantly longer start up times.